1. Field of the Invention
The present invention relates to an air/fuel (A/F) ratio sensor for achieving appropriate control of combustion in internal combustion engines and other combustors by means of detecting the concentration of oxygen in the exhaust.
2. Background Art
With a view to improving fuel economy and reducing emissions, there is increasing use of automatic combustion control devices for maintaining optimum conditions of combustion in combustors, especially automotive engines. Such devices must be supplied with the necessary control information from external sources. For attaining this purpose, there have been developed various types of A/F ratio sensors which are capable of detecting the air/fuel (A/F) ratio (or excess air ratio, .lambda.) of an air/fuel mixture by means of measuring the concentration of oxygen or inflammable components in the exhaust gas. The excess air ratio is related to the air/fuel ratio by the expression .lambda.=(A/F)/14.7.
FIGS. 1 and 2 show the structure of a conventional A/F ratio sensor of the type which employs an oxygen concentration electrochemical cell. Since it is intended for use within an atmosphere of interest, say, the exhaust system of an automotive engine, this sensor has an oxygen pump A and an oxygen concentration electrochemical cell B, both of which have an elongated plate-like contour and which are spaced apart from each other by a small closed space a that is provided with a hole (or opening) communicating with the atmosphere of interest. The oxygen pump A and the oxygen concentration electrochemical cell B are almost alike in structure. In the device shown in FIGS. 1 and 2, a plate 1 (or 4) measuring 4 mm wide, 40 mm long and 0.7 mm thick is made of an oxygen-ion conductive solid electrolyte such as zirconium dioxide (ZrO.sub.2 partially stabilized with Y.sub.2 O.sub.3). Opposite sides of the tip of the plate 1 (or 4) are coated with electrode layers 2 and 3 (or 5 and 6), for example, of porous platinum formed by an appropriate thick-film deposition technique. The porous platinum is in the form of platinum paste that measures 2 mm wide, 3 mm long and about 20 micrometers thick and which has a porosity of about 30% and a binder content of 20 wt %. The electrodes 2 and 3 (or 5 and 6) are connected to lead wires 8 and 9 (or 10 and 11), respectively, and leads 8 and 11 are grounded through a terminal 14 while the leads 9 and 10 are separately led out through terminals 14 and 15.
The oxygen pump A is fed with d.c. power from a source 30 through a variable resistor 31 which is capable of regulating the current flow to the oxygen pump A. The oxygen concentration electrochemical cell B is connected via lead 10 to a voltmeter 32 for output measurement. In FIG. 1, a metal fixture 20 assists in the installation of the sensor. Various insulators 21-24 isolate the conductors. A sheath 27 facilitates the installation of the terminals. A cover 25 (FIGS. 1 and 2) protects the pump A and the cell B. A nail-shaped fin 26 is provided inwardly at a plurality of positions on the periphery of the cover 25 and serves as a hole communicating with the atmosphere of interest. A ceramic heater 7 is in a plate form. A power source 33 provides power for the heater 7 through a wiring terminal 16.
The sensor shown in FIGS. 1 and 2 is operated in the following manner. When a switch (not shown) is turned on, a constant current whose level is predetermined by the variable resistor 31 is applied from the constant-voltage d.c. supply 30 to both electrodes 2 and 3 on the oxygen pump A. The oxygen pump A then ionizes the oxygen in the atmosphere in the small closed space a (gap width=0.1 mm). The negative electrode 2 is in contact with the gap a and thus provides electrons to the oxygen. The ionized oxygen is transmitted through the tabular layer 1 of solid electrolyte toward the positive electrode 3. The oxygen ions reaching the positive electrode surface 3 are deprived of electrons and released into the atmosphere of interest as molecular oxygen. This is the pumping-out mechanism achieved by the oxygen pump A. As oxygen is removed from within the small closed space a, the oxygen in the atmosphere of interest flows into the space a by diffusion through the communication hole 26, to thereby create an equilibrium within the space a. In the space a, oxygen remains at a given concentration which is determined in accordance with the concentration of oxygen in the atmosphere of interest.
In the electrochemical cell B, one electrode surface 6 is in contact with the space a and the other electrode surface 5 is in contact with the atmosphere of interest or any atmosphere such as air having a reference oxygen partial pressure. The generated EMF is then detected and the concentration of oxygen in the atmosphere of interest is thereby detected for use as a basis for calculating the A/F ratio of that atmosphere. An optimum current value may be determined for specific conditions of use of the sensor by preliminary operational testing with the combustor.
If an A/F ratio sensor having the structure described above is used i an automotive engine, it frequently occurs that the solid electrolyte zirconium oxide 1 blackens in the vicinity of the electrode 2 on the oxygen pump A which is in contact with the small closed space a. This phenomenon called "blackening" is irreversible in that it will not disappear if it has progressed too far. The present inventors found that as blackening proceeds, the internal resistance of the oxygen pump also increases, and that if the degree of blackening increases too much, fine cracks will develop in the solid electrolyte 1 and grow in both size and number until the pump fails completely.